Re: domain creation vs querying free memory (xend and xl)

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Wed, Sep 26, 2012 at 02:17:06PM -0700, Dan Magenheimer wrote:
> I was asked a question that seems like it should be obvious
> but it doesn't seem to be, at least in xm-land.  I'll look
> into it further, as well as for xl, but I thought I'd ask
> first to see if there is a known answer or if this is a known
> problem:
> 
> Suppose that xm/xl create is issued on a large-memory
> domain (PV or HVM or, future, PVH).  It takes awhile
> for this domain to launch and during at least part of this
> time, the toolset hasn't yet requested all of the
> required memory from the hypervisor to complete the
> launch of the domain...  or perhaps the toolset has,
> but the hypervisor is slow about calling the long sequence
> of page allocations (e.g. maybe because it is zeroing
> each page?).
> 
> Then it is desired to launch a second large-memory domain.
> The tools can query Xen to see if there is sufficient RAM
> and there is, because the first launch has not yet
> allocated all the RAM assigned to it.
> 
> But the second domain launch fails, possibly after
> several minutes because, actually, there isn't enough
> physical RAM for both.
> 
> Does this make sense?  Should the tools "reserve"
> maxmem as a "transaction" and/or ensure that "xm/xl
> free" calls account for the entire requested amount
> of RAM?  Or maybe xl _does_ work this way?

So say "freeze" the amount of free memory. Lets CC the XCP folks
> 
> Thanks for any comments or discussion!
> 
> Dan